Continuous process for production of steel part with regions of different ductility

ABSTRACT

The interior of a furnace is partitioned into two longitudinally extending and transversely adjacent zones, and one of the zones is heated to a substantially higher treatment temperature than the other of the zones, which may or may not be heated. A steel workpiece is conveyed longitudinally through the furnace with a region of the workpiece moving exclusively through the one zone and another region of the workpiece moving exclusively through the other of the zones such that the regions are heated to different temperatures. The treatment temperature in one of the zone is above the AC 1  point for the workpiece and the temperature in the other of the zones is close to or below the AC 1  point for the workpiece.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and apparatus forheat-treating a steel workpiece. More particularly this inventionconcerns the production of a steel part having regions of differentgrain structure and/or ductility.

BACKGROUND OF THE INVENTION

[0002] It is known to make various motor-vehicle parts—e.g. tie rods,B-columns, struts, door beams—of hardened steel with uniform ductilityand grain structure throughout the entire workpiece. This isaccomplished by a heat treatment of the part, raising it to apredetermined temperature and then quenching it in accordance with thedesired characteristics of the finished product. The main factoraffecting grain structure and/or ductility is the maximum temperature towhich the workpiece is heated, that is whether or not it reaches any ofseveral critical temperatures, the so-called AC₁-AC₄ points.

[0003] For specific parts, however, it is desirable for the grainstructure to vary from one region to another. One region might need tohave exceptional strength while another might need to be able to deformsomewhat. This can be accomplished most simply by making the part as acomposite of two pieces that are differently treated to have the desiredcharacteristics.

[0004] U.S. Pat. No. 5,972,134 describes a one-piece part having regionsof different ductility. It is produced by heating it locally beforedeforming it into the desired shape and then cooling it. Thedisadvantage of this method is that it is a complex batch operation thatdoes not lend itself to the mass production needed for motor-vehiclemanufacture.

[0005] German utility model 200 14 361 published 16 Nov. 2000 describesa door post or so-called B-column that is rendered austenitic in afurnace and then is simultaneously deformed and quenched in a die. Someparts of the workpiece are insulated before it is put in the furnace sothat they do not become austenitic and thus do not when hardened developa martensitic grain structure. Such a process is also unwieldy,involving the application and removal of insulation before and after theheat treatment, two extra steps that considerably elevate the cost ofthe workpiece.

[0006] For mass production of parts it is standard to use a continuousfurnace through which the parts move along a path on a conveyor. U.S.Pat. No. 4,622,006 describes such a continuous-heating furnace which isprovided with means for taking out and inserting workpieces at severallocations along the treatment path. Thus it is possible, with acontinuously operating furnace, to differently heat treat differentworkpieces, some staying in the furnace for the full treatment andothers only being heated for substantially less time. While this systemis indeed very flexible, it does not allow one to produce a singleworkpiece having regions that are treated differently.

OBJECTS OF THE INVENTION

[0007] It is therefore an object of the present invention to provide animproved system for producing a workpiece with regions of differentgrain structure and/or ductility.

[0008] Another object is the provision of such an improved system forproducing a workpiece with regions of different grain structure and/orductility which overcomes the above-given disadvantages, that is whichallows such workpieces to be produced in a simple and continuous processthat lends itself to a low-cost mass-production operation.

[0009] SUMMARY OF THE INVENTION

[0010] According to the invention the interior of a furnace ispartitioned into two longitudinally extending and transversely adjacentzones, and one of the zones is heated to a substantially highertreatment temperature than the other of the zones, which may or may notbe heated. A steel workpiece is conveyed longitudinally through thefurnace with a region of the workpiece moving exclusively through theone zone and another region of the workpiece moving exclusively throughthe other of the zones such that the regions are heated to differenttemperatures.

[0011] This method can be carried out on an unhardened workpiece or onone that has already been hardened. In the former case, part of theworkpiece is hardened and part of it is either left untreated, orhardened less. In the latter case a part of the workpiece is heatedsufficiently to soften or anneal it, and the remainder is left in itshardened condition. Either way, the result is a workpiece with adjacentregions of different hardness/ductility.

[0012] The treatment temperatures in the two furnace zones are selectedaccording to the desired workpiece characteristics. When for example amotor-vehicle B-column is being made its foot should be quite ductile sothat, in an accident, it can bend at its lower end without breaking off.The shaped part is thus moved through the furnace with its foot in thelower-temperature zone. The zone with the foot is maintained below theAC₁ point (the temperature at which austenite begins to form) and thezone with the rest of the post is maintained above the AC₃ point (thetemperature at which all the ferrite has been transformed to austenite).After heat-treatment according to the invention, the workpiece issubjected to the normal hardening steps, e.g. simultaneous quenching andclamping in a die. Thus the foot in the other zone is left ductile, asits grain structure will remain substantially unchanged, while thebalance of the post is very strong.

[0013] According to the invention the treatment temperature in one ofthe zones can be above the AC₁ point for the workpiece and thetemperature in the other of the zones below the AC₁ point for theworkpiece. This is ideal for a workpiece, e.g. a strip, that has beenpartially hardened before the zone-wise treatment according to theinvention.

[0014] It is also possible simply to leave the other zone unheated. Thisis done when the workpiece has its final shape and merely needs to behardened in one region. The other zone is thus generally at ambienttemperature, below anything that would affect the grain structure ofsteel.

[0015] Also according to the invention the other zone is heated tobetween the AC₁ point and the AC₃ point of the workpiece and the onezone is heated to above the AC₃ point of the workpiece. Thus there ispartial conversion in the low-temperature zone but completegrain-structure conversion in the high-temperature zone.

[0016] Steel that has a carbon content greater than 0.8% is treated inanother system of this invention where the other zone is heated toslightly below the AC₁ point of the workpiece and the one zone is heatedto slightly above the AC₃ point of the workpiece. Thus thelow-temperature region is annealed and its grain structure is relaxed.

[0017] In order to prevent oxidation of the workpiece, an inert gas isinjected into the furnace. The gas can be, for example, nitrogen.

[0018] An apparatus for heat-treating a steel workpiece according to theinvention thus has a longitudinally extending furnace subdividedinternally by partitions into at least two longitudinally extending andtransversely adjacent zones. Means such as electrical coils or burnersare provided for heating one of the zones to a substantially highertreatment temperature than the other of the zones. A conveyor transportsthe workpiece longitudinally through the furnace with a region of theworkpiece moving exclusively through the one zone and another region ofthe workpiece moving exclusively through the other of the zones suchthat the regions are heated to different temperatures.

[0019] The furnace normally is of the tunnel type, with a longitudinallythroughgoing conveyor made of rollers. The workpieces push each otherthrough the furnace, or the rollers are rotated to advance them.Alternately the furnace can be of the carousel type with an annular pathfor the workpieces. No matter what the shape of the oven, the partitionextends parallel to the path of movement of the workpieces through it.

[0020] The partitions in accordance with the invention include alongitudinally extending upper partition above the transport means and alongitudinally extending lower partition below the transport means andvertically aligned with the upper partition. The upper and lowerpartitions define a transversely open gap through which the transportmeans and the workpiece extend.

[0021] It is possible for one of the partitions to be displaceabletransversely through a plurality of different transversely offsetpositions. This way the furnace can accommodate differently proportionedworkpieces. In this arrangement there can be a plurality of the lowerpartitions transversely offset from each other and the upper partitionis displaceable transversely through positions aligned with each of thelower partitions.

[0022] The partitions can also include a middle longitudinally extendingpartition aligned vertically between the upper and lower partitions. Theconveyor means transports the middle partition through the furnace withthe workpiece. Thus a complex three-dimensional workpiece can be treatedwithout excessive heat leakage between the furnace zones. This system isparticularly applicable to a furnace where the workpiece is carried on asupport, typically a plate.

BRIEF DESCRIPTION OF THE DRAWING

[0023] The above and other objects, features, and advantages will becomemore readily apparent from the following description, reference beingmade to the accompanying drawing in which:

[0024]FIG. 1 is a largely schematic end view of a furnace for carryingout the method of this invention; and

[0025]FIGS. 2 through 4 are further views like FIG. 1 of other furnacesin accordance with the invention.

SPECIFIC DESCRIPTION

[0026] As seen in FIG. 1 a furnace 1 is internally subdivided by apartition 2 into two zones 1 a and 1 b. A B-column 3 is moved throughthis furnace 1 in a direction perpendicular to the plane of the view andis positioned such that one region 3 a lies in the zone la and anotherregion 3 b lies in the zone 1 b, the partition 2 of course being slottedto allow the workpiece 3 to be thus positioned. The workpiece 3 is madeof hardenable steel with an AC₁ point (the temperature at whichaustenite begins to form) of 740° C. and an AC₃ point (the temperatureat which all the ferrite has been transformed to austenite) of 850° C.It is heated in the zone la to a temperature of about 700° C. and in thezone 1 b to a temperature of about 950° C. The workpiece 3 issubsequently worked and ends up having substantially greater strength inthe region 3 b and greater ductility in the region 3 a.

[0027]FIG. 2 shows a furnace 4 having a roof 4 a from which hangs ashort vertical partition 5 a and a floor 4 b on which stands anothershort vertical partition 5, with conveyor rollers 6 extending throughthe gap between the two partitions 5 and 5 a. A workpiece 7 is supportedon the rollers 6 that are rotated as indicated by the arrow to advanceit horizontally through the furnace 4, with one region 7 a in a zone 4 dto one side of the partitions 4 and 4 a and another region 7 b in a zone4 c to the opposite side. The zone 4 c is cooler than the zone 4 d andheat is mainly applied to the zone 4 d.

[0028] The furnace 8 of FIG. 3 has a top wall 8 a with a short dependingpartition 9, a floor 8 d supporting a short partition 9 b and rollers 10that advance a three-dimensional workpiece 12 on a support 11. In orderto fill the large gap between the partitions 9 and 9 b, a shortpartition 9 a is carried on the support 11 and moved through the furnace8 with the workpiece 12. Once again, the temperature is different in thezones 3 b and 3 c defined to opposite sides of the partitions 9, 9 a,and 9 b.

[0029] The furnace 13 of FIG. 4 has a single upper partition 14depending from a roof 13 and extending parallel to the transportdirection which once again is perpendicular to the plane of view. Threefurther partitions 14 a, 14 b, and 14 c extending parallel to each otherand spaced transversely stand on a floor 13 b of the furnace 13, withthe partition 14 a coplanar with the partition 14. Conveyor rollers 15advance workpieces 16 through the furnace 13. Dashed lines 16 a and 16 bshow how, instead of planar workpieces 16, three dimensional workpiecescan be accommodated.

[0030] Thus the furnace is subdivided longitudinally into a compartment20 a to one side of the two partitions 14 and 14 a and a compartment 20b to the other side thereof, with the workpiece 16 extending between thechambers. Heaters 17 on the roof 13 a and 17 a on the floor 13 b heatthe chambers 20 a and 20 b to different temperatures. The upperpartition 14 a can be moved into alignment with either of the partitions14 b or 14 c, to which ends gaps 19 and 19 a are formed in the roofheater 17, so as to accommodate differently shaped workpieces.

[0031] After the workpiece is taken out of any of the above-describedfurnaces it is typically subjected to a hot-working process or otherwisehardened. The result is that the region treated at lower temperaturewill have radically different ductility and/or hardness than the regionof the workpiece treated at the higher temperature in the furnace. In asituation where one region of the workpiece is not subjected to anyfurther hardening operation, its zone of the furnace is in fact leftunheated.

We claim:
 1. A method of heat-treating a workpiece of hardenable steel,the method comprising the steps of: partitioning the interior of afurnace into two longitudinally extending and transversely adjacentzones; heating one of the zones to a substantially higher treatmenttemperature than the other of the zones; and conveying the workpiecelongitudinally through the furnace with a region of the workpiece movingexclusively through the one zone and another region of the workpiecemoving exclusively through the other of the zones such that the regionsleave the furnace at different temperatures.
 2. The heat-treatmentmethod defined in claim 1 wherein the treatment temperature in one ofthe zones is above the AC₁ point for the workpiece and the temperaturein the other of the zones is below the AC₁ point for the workpiece. 3.The heat-treatment method defined in claim 1 wherein the other zone isnot heated.
 4. The heat-treatment method defined in claim 1 wherein theother zone is heated to between the AC₁ and AC₃ points of the workpieceand the one zone is heated to above the AC₃ point of the workpiece. 5.The heat-treatment method defined in claim 1 wherein the other zone isheated to slightly below the AC₁ point of the workpiece and the one zoneis heated to slightly above the AC₃ point of the workpiece.
 6. Theheat-treatment method defined in claim 1 wherein the workpiece isunhardened prior to conveyance through the furnace, the other zone beingheated sufficiently to harden the other region of the workpiece.
 7. Theheat-treatment method defined in claim 1 wherein the workpiece ishardened prior to conveyance through the furnace, the other zone beingheated sufficiently to anneal and soften the other region of theworkpiece.
 8. The heat-treatment method defined in claim 1, furthercomprising the step of injecting an inert gas into the furnace andthereby preventing oxidation of the workpiece.
 9. An apparatus forheat-treating a steel workpiece, the apparatus comprising: alongitudinally extending furnace; partition means subdividing aninterior of the furnace into two longitudinally extending andtransversely adjacent zones; means for heating one of the zones to asubstantially higher treatment temperature than the other of the zones;and transport means for conveying the workpiece longitudinally throughthe furnace with a region of the workpiece moving exclusively throughthe one zone and another region of the workpiece moving exclusivelythrough the other of the zones such that the regions are heated todifferent temperatures.
 10. The heat-treatment apparatus defined inclaim 9 wherein the partition means includes a longitudinally extendingupper partition above the transport means and a longitudinally extendinglower partition below the transport means and vertically aligned withthe upper partition, the upper and lower partitions defining atransversely open gap through which the transport means extends.
 11. Theheat-treatment apparatus defined in claim 10 wherein at least one of thepartitions is displaceable transversely through a plurality of differenttransversely offset positions.
 12. The heat-treatment apparatus definedin claim 11 wherein there are a plurality of the lower partitionstransversely offset from each other and the upper partition isdisplaceable transversely through positions aligned with each of thelower partitions.
 13. The heat-treatment apparatus defined in claim 12wherein the partition means includes a middle longitudinally extendingpartition aligned vertically between the upper and lower partitions, thetransport means displacing the middle partition through the furnace withthe workpiece.